There is a need for image sensors to be sensitive to ultraviolet and far ultraviolet radiation. Image sensors of the frame transfer type cannot be used with frontside illumination due to the absorption of the short wavelength radiation by polysilicon gate electrodes. Interline type imagers, while sensitive to the radiation, provide only 30-35% of their surface area for sensing, and are thereby limited.
Presently, a number of investigators are pursuing the development of backside illuminated image sensors by removing silicon from the backside of the devices. The thinned backside is treated to provide an electrical potential which forces electrons generated at or near the backside towards potential wells under the image gates. See J. Janesick et al. "CCD Pinning Technologies", SPIE Optical Sensors and Electronic Photography, Vol. 1071-15, Los Angeles, Jan. 16-18, (1989); C. M. Huang et al. Abstract No. 481, 174th Electrochemical Society Mtg., 88-2, 705 (1988); T. W. Edwards and R. S. Pennypacker, U.S. Pat. No. 4,266,334, May 21, 1981; and M. Blouke et al., Optical Engineering, Vol. 26, No. 9, 837, (1987). These methods involve operations which can degrade the silicon quality, such as mechanical lapping, ion implantation, and laser annealings. The loss of silicon quality results in high surface recombination velocity values. To shield the back surface from recombination losses, very high doping gradients are required to form an adequate backside electrical potential. See R. A. Stern et al., Optical Engineering, Vol. 26, No. 9, 875 (1987) and C. Huang, Proceedings 1991 IEEE CCD Workshop, University of Waterloo (1991).
A technique for bonding quartz wafers to silicon wafers has been reported. See G. Goetz and A. Fathimulla, Abstract No. 309, 177th Electrochemical Society Meeting, 90-1, 462 (1990). This method requires that the silicon bonded to the quartz be etched into islands prior to high temperature processes in order to avoid damage associated with thermal expansion induced stress.
Methods for providing silicon etch stops have been disclosed. For example, see W. P. Maszara et al., J. Applied Physics, 64(10) 4943 (1988); H. Seidel et al, J. Electrochemical Society, Vol. 137, No. 11, 3626 (1990); and H. Muraoka et al. "Controlled Preferential Etching Technology", in Semiconductor Silicon 1973, edited by H. R. Huff and R. R. Burgess, Electrochemical Society, Princeton, N.J. 327, (1973).
The diffusion of boron into silicon from doped oxide surfaces has been modeled by Barry. See Barry and Olofsen, J. Electrochemical Soc., Vol. 116, No. 6, 854, (1969) and Barry and Manoliu, J. Electrochemical Society, Vol. 117, No. 2, 258, (1970).
Calculations of image sensor quantum efficiency have been made by several groups. For example see: C. M. Huang et al., "Future Development for Thinned Back-Illuminated CCD Imager Devices", 1991 IEEE CCD Workshop, University of Waterloo, Canada (1991); J. Janesick and D. Campeu, IEEE Int. Electron Devices Meeting, 350 (1986); M. M. Blouke, Proc. SPIE, 1439 (1990); and R. Stern et al. in Instrumentation in Astronomy VI, D. C. Crawford, Ed., Proc. SPIE 627, 583 (1986).
The deposition and properties of borosilicate glasses have also been studied. See W. Kern and R. C. Heim, J. Electrochemical Soc., Vol. 117, No. 4, 562 and 568, (1970). The use of a borosilicate glass layer to support a silicon wafer undergoing a thinning process is reported in U.S. Pat. No. 4,946,716, Aug. 7, 1990 to Brian L. Corrie.
Existing fabrication methods thin the device wafer after the devices have received most of the steps involved in their manufacture. Yield losses incurred due to thinning and backside accumulation operations are expensive since a great deal of processing has been invested in the devices prior to thinning. In addition, some methods require contacting of the device bond pads from the wafer backside. This involves aligning backside masks to the frontside device structures and etching contact holes to the bond pads. This is a complex and expensive process.